1. Field of the Invention
The present invention pertains to a method and system for reducing vapor emissions from floating roof storage tanks for petroleum liquids using a vapor-suppressing foam.
2. Background
Floating roof type storage tanks are commonly used to store petroleum liquids to minimize the generation of vapors and alleviate problems associated with venting the tanks during filling and emptying. However, floating roof type storage tanks typically include some fixed interior space which is created when the floating roof descends to its minimum height dictated by support legs which are typically connected to the roof or extend upward from the tank bottom. In order to minimize the generation of vapors during a change of the type of liquid stored in a conventional floating roof tank, the interior tank space created by the displacement of one petroleum liquid during emptying is taken up by pumping water into the tank. When the new and different petroleum liquid is pumped into the tank, the water is displaced from the tank and must then be treated to remove petroleum contaminants and otherwise render the water suitable for disposal. A typical floating roof type storage tank may generate as much as one hundred thousand (100,000) cubic feet of waste water during the changeover from one type of liquid previously stored to the new liquid to be stored.
One promising and desirable method of minimizing the generation of vapors in the handling and storage of petroleum liquids pertains to the use of a foam which may be placed on top of the liquid being stored or may be pumped into the tank space to be occupied by the liquid to prevent the generation and accumulation of volatile and hazardous vapors. U.S. patent application Ser. No. 07/653,398, filed Feb. 11, 1991 in the name of Thomas K. Perkins and assigned to the assignee of the present invention, describes a method for controlling vapor emissions during the loading of marine tankers with hydrocarbon liquids wherein a blanket of aqueous foam is placed in the fixed volume compartments of the tanker to form a barrier between the hydrocarbon liquid and the tank space above the liquid to minimize the generation of volatile vapors in the tank compartments. U.S. Pat. No. 3,850,206 describes an earlier method of introducing a vapor-suppressing foam to tanker cargo tanks.
The present invention eliminates the need to handle large volumes of hazardous waste water generated in the change of liquids in floating roof type storage tanks using a type of foam similar to that described in the abovementioned patent and patent applications. Certain improvements in foam composition such as described in U.S. patent application Ser. No. 07/584,978, filed Sep. 19, 1990 in the name of Sophany Thach et al, and assigned to the assignee of the present invention, and also as described in International Patent Application Number PCT/US 91/06795 also assigned to the assignee of the present invention may also be used with the method and system of the present invention.